Add lockfile per namespace

Attempt to create a lock file per namespace on initialization before
creating spool file directories. Error out if there is already a lock
file in place (which means the current process is a duplicate)

Update tests to reflect this.

Also updated existing tests to call marquise_shutdown, where applicable,
to cleanup locks from tests.

Addresses issue #23 
This is safe but not memory efficient. `sizeof` returns bytes and `<<` shifts by bits. `(sizeof(pid_t) * 8)` will give the number of bits needed for a pid, and you can turn that into the exact value for decimal digits using logs.  I don't have a problem leaving it as-is, but maybe add a comment that it overestimates memory required?